Amortentia - "Amor" is the Latin word for "love," and "tentia" is derived from "tentare," which means "the handling of," "the making of an attempt," or "the attack on." Hence, "the handling of love," "making an attempt to love," or "the attack on love."

Aparecium (Spell that makes invisible ink appear) - From the Latin word "aperio," meaning to "uncover, lay bare, reveal, or make clear" or "apparere," meaning to "make clear." It is spelled with only one "p," perhaps because of "apertus" which means "open, obvious, public."

Arithmancy - A method of fortune-telling based on names, numbers, and mathematical calculations. From the Greek, "arithmo" meaning "number" and "mancy" meaning "prophecy." It is also known as numerology.

Arresto Momentum (Spell that stops the descent of a falling object) - This is strictly a movie-based spell and literally means "stop movement."

Avada Kedavra (Killing Curse) - Aramaic phrase that means "I will destroy as I speak." Also similar to "Abra-cadabra", which is an ancient spell (dates from the 2nd Century) used by conjurors to invoke spirits or supernatural powers for protection against disease or aid. "Kedavra" sounds like "cadaver," which means "corpse."

Avis (Spell that Ollivander used to make birds fly out of Krum's wand) - Latin for "bird."

B
Bezoar - A bezoar is indeed "a ball of indigestable material that can be found in the stomach of certain animals," most notably the so-called bezoar goat (capra aegagrus). And indeed it was believed in ancient times that a bezoar could serve as an antidote for most poisons.
Bluebottle (Make of broomstick) - A type of annoying fly with a loud buzz and iridescent body. Also a small, blue jellyfish (also known as a Man-O-War). They appear on beaches after strong winds and their sting is very painful.

Boomslang - One of the ingredients used in brewing Polyjuice Potion, a boomslang is actually a South African snake. Boomslangs live in trees and bushes and feed on small animals and bird eggs. They are greenish to brown or black in color and grow to about 1.5 m (about 5 feet) long. Most members of the family (Colubridae) to which the boomslang belongs are harmless, but the boomslang has a potent venom that it delivers through large, deeply grooved fangs that are located at the rear of the mouth. The bite of the boomslang can be fatal.

Bubotuber pus - "Tuber" refers to the fact that the bubotuber is a plant, which extends perpendicularly into the soil. It's pus is dangerous to the skin. "Bubos" is an English word for an inflamed, tender swelling of a lymph node, especially in the area of the armpit or groin. It is characteristic of certain infections, such as

Conjunctivitus Curse (Spell that Krum used to "do something" to the eyes of the Chinese Fireball during the First Task of the Triwizard Tournament) - Conjunctivitis is the scientific name for pink-eye -- the illness that children often get that makes their eyelids crust together.

D
Deletrius (Banishing Spell) - Latin for "that which is erased."

Densaugeo (Spell that Malfoy used to make Hermione's front teeth grow enormous) - "Dens" is Latin for "teeth." "Augeo" is Latin for "I enlarge."

Expecto Patronum (Spell used to conjure a Patronus) - In Latin, "expecto" is to "await, desire, or hope for" and "patronus" is "protector." Hence, "to hope for a protector." A Patronus is used to protect against a Dementor.

F
Felix Felicis (Luck Potion) - "Felix" is Latin for "lucky, fortunate, or happy." "Felicis" is derived from two Latin adjectives, one for "lucky" and one for "of the lucky." It translates as "lucky of the lucky," but seems more acceptable to write it as "luck of luck." Could haves ties to the word "felicity" which means "extreme happiness."

Ferula (Spell that creates a splint or wooden rod) - From the Latin word "ferula," meaning a "rod to beat children with." In Spanish, the word "ferula" refers to an object used to immobilize a limb, like a broken leg. The object can be either a stick to tie to the limb or a cover of plaster.

Fidelius (Charm that makes someone a Secret-Keeper) - In Latin, "fidelis" is the comparative form of "fidelis." Thus, meaning "a person who is more faithful, devoted, loyal, earnest, true, trustworthy, dependable, reliable, constant or lasting."

Fiendfyre – literally translates, fiend as enemy, fyre as an Old English spelling of fire.
Finite Incantatem (Spell that cancels out other spells) - "Incantatem" could be related to the Latin "Incantationem," which means "incantation." Together the phrase translates as "Stop the incantation!"

G
H
Horcrux - "Horcrux" when broken down in many languages means "outside the cross." This is consistent with the very unholy nature of creating one, and why it is stricken from the pages in a lot of textbooks. "Crucis" means "pain or torture," and "hor" is a shortened form of the noun "horreum," which means "storehouse." Thus, "tormenting storehouse." A Horcrux is effectively a "storehouse" for the part of the soul that an individual destroys when killing someone. "Hor" can also remind readers of the words "horrible" and "horrid." The English meaning for "crux" is "the critical feature or essence," like the crux of an argument. Similar to the Latin translation, it then becomes understood as "essence storehouse." Many consider the soul to be the essence of an individual. A "crux" is also defined as a "difficult puzzle," so Horcruxes can then be seen as "horrible" or "tormenting puzzles." In Egyptian mythology, Horus was the son of the god Osiris, who became the God of the Dead. Crux is also Latin for "cross." If you combine these two words, you get the "cross of Horus," also known as the "ankh" (a cross with a loop at the top). The ankh was the symbol of life. Thus, a Horcrux would ensure life.

I
Impedimenta (Spell used to slow down attackers) - "Impedio" is Latin for "I hinder." In Latin, "Impedimenta" means "obstacle," as in creating an obstacle to impede one's path or goal.

Liberacorpus (Spell that releases or frees the individual suspended in mid-air) - In Latin, "libera" means "to free" and "corpus" means "body." Combining these words translates as to "free the body." Counter-curse to Levicorpus.

Lumos (Spell used to make a wand emit light) - "Lumen" is Latin for "light" and "luminous" means "emitting light" in English.

M
Mandragora - In medieval times, Mandragora or mandrake root was though to have magical properties. It was thought to resemble the human figure and was known to cause sleepiness.

Mirror of Erised - Erised backwards is desire (as in "you'll see what you desire"). The inscription around the top of the Mirror of Erised, if shown backwards with the spaces rearranged, says: "I show not your face, but your heart's desire." Oddly enough, Eris was the Greek goddess of strife.
Morsmordre (Makes the Dark Mark appear in the sky) - Combination between "mors" (Latin for "death") and "mordere" (Latin for "to bite"). Death bite? No -- Death Eater.

O
Obliviate (Spell that erases one's memory) - Used in Chamber of Secrets when Lockhart tries to wipe out Ron's and Harry's memories. Comes from the word "obliterate," meaning to "wipe out, erase, or remove all traces." Also sounds like "oblivious" meaning "forgetful."

Oppugno (Spell that makes conjured creatures attack on the command of the conjurer) - In Latin, means "I attack."

Orchideous (Spell that makes flowers spring out of the tip of a wand) - An orchid is a type of flower.

P
Patronus (Charm used in defense against Dementors) - "Patronus" is Latin for "protector."
Pensieve - The verb "penser" in French means "to think." Perhaps a combination of the English words "pensive" and "sieve." To "be pensive" is to be "wistful or thoughtful" and "a sieve" is "a utensil of wired mesh used for sifting."

Protean Charm - Most likely named after Proteus, a Greek god who could change his shape at will. Hermione uses this charm to alert members of Dumbledore's Army of future meetings in Order of the Phoenix.

Q
Quibbler - To "quibble" means to "evade the truth or importance of" an issue by raising trivial distinctions and objections. A "quibble" is an archaic term for a "pun." A term used to describe Ancient Greek philosophers. The philosophers were referred to by the commoners as "quibblers" and the act of philosophizing was known as "quibbling."

Quidditch - J.K. Rowling has stated that the origin of this name is entirely made up (she wrote five pages of "Q" words until she found one that she liked), but it is still interesting to the note that the word "quiddity" means "the essence or real nature of a thing."

Quietus (Spell that cancels out "Sonorus") - Obviously "quiet" means to be "silent", and "quietus" in Latin means to be "quiet."

R
Reducio (Shrinking Charm) - "Reduce" means "to make smaller" in English and also "Reduco" in Latin means "to reduce."

Rennervate (Spell used to revive someone who has been stunned) - J.K. Rowling originally changed the name of this spell from Ennervate. Possibly from the French "revenir" meaning "to return." Also means to "add nerve" (daring or strength). Sounds similar to "re-energize."

Riddikulus (Spell used to transform a scary boggart into a humorous shape) - "Ridiculous" means "absurd."

S
Sectumsempra (Spell used to seriously cut another person) - In Latin, "sectum" means to "cut, wound, or amputate" and "sempra" is derived from the word "semper" meaning "always" or "at all times." Hence, "to wound always or make a permanent wound."